91 



On this view the appearance of the /3 secondary ray would be 

 really a scattering of the incident ray ; and this would make 

 the 7 I'ay fall into line with other radiations whose secondary 

 radiations are either scattered primary or o rays. 



If the gradual disappearance of a stream of y radiation were 

 caused by collision in this way, the number disappearing in 

 any unit of length of the course would be proportional to 

 the total number in the stream, so that an exponential law 

 would result. 



It appears, therefore, that all the known properties of 

 the y rays are satisfied on the hypothesis that they consist 

 of neutral pairs. 



It is interesting to carry the speculation a little further, 

 and to observe that a pair possessing a very circumscribed 

 field might exhibit no ionisation effects whatever, and be 

 capable of very great penetration. Its final end might be an 

 incorporation into an atom traversed (as has been suggested 

 to me by Professor Rutherford in reference to the a particle). 

 Thus, penetrating radiation of this kind might exist in some 

 quantity without our being aware of it, and might be an 

 important agent in the breaking down and building up of 

 atoms. 



If we attempt to explain the properties of the X-ray on 

 the supposition that it is a neutral pair, we meet with a diffi- 

 culty which does not occur in the case of the y ray. For it 

 has been shown by Marx (Phys. Zeit., p. 268, 1905) that 

 certain X-rays move with the velocity of electric waves in 

 wires, and therefore of light. Now, it is difficult to conceive 

 that material particles can move with such a speed and yet 

 be scattered on impact with atoms. Yet in other ways the 

 behaviour of X-rays is so consistent with what we should 

 expect on a neutral-pair theory that it does not seem either 

 useless or uninteresting to consider the matter from that 

 point of view. 



In the first place we have the necessary absence of de- 

 flection in electric and magnetic fields, and of refraction; 

 the possibility of a kind of polarisation quite different in 

 character to that of light; great penetration, whose amount 

 may vary with the moment of the pair, or with the velocity if 

 the latter is a variable; the production of secondar}^ rays 

 when the ray strikes an atom, with an easy explanation of 

 why the ray, when striking a light and yielding atom, is 

 reflected more or less unchanged, yet, striking a heavy 

 atom, is shattered with the production of much easily ab- 

 sorbed secondary radiation of the cathode ray type (Barkla, 

 Phil. Mag., June, 1906). If the X-ray is an ether pulse it 

 is difficult to understand, as Thomson has shown (Conduction 



